simplified iterator implementation

Removed `$token` and `$index` used in `Iterator` interface - since `$benchmarks` is an array - internal pointer can be used instead.
The empty() on function results not compatible with PHP 5.4
```
PHP Fatal error:  Can't use method return value in write context in /home/travis/build/fabfuel/prophiler/src/Fabfuel/Prophiler/Profiler.php on line 86
```